The present invention relates to an electronic circuit for S correction of the vertical scanning line in a television apparatus.
As is known, image synthesis in television is produced with an electron beam of which is modulated by the video signal.
Scanning of the beam over the whole screen is obtained by a combination of magnetic fields produced by horizontal and vertical deflection coils arranged on the neck of the kinescope.
In the case of vertical deflection the magnetic field must be such that it produces deviations of the beam from the top of the screen toward the bottom thereof and then return rapidly to the starting point for the next frame period.
The current running through the vertical deflection coil in a frame period is approximately a saw-tooth function. Consideration that the television screen is generally flat rather than round so that the spaces covered by the electron beam are proportionate to the tangent of the scanning angle rather than to the angle itself, to hold scanning speed steady both in the middle and at the edges of the screen it is necessary to introduce in the saw-tooth function an appropriate correction capable of reducing the slope of the ramp in the vicinity of the minimal and maximal values. This type of correction is commonly called "S correction".
A known type of ramp generator includes a direct current generator, a capacitance charged by the direct current generator, and a switch controlled by the vertical synchronism signal to periodically discharge said capacitance.
To perform the S correction of the scanning ramp thus produced the commonly used solution calls for employment of two distinct capacitances in series with usually the same value (0.1 u F), to the common terminals of which the voltage ramp taken at the generator output is brought back through a feedback circuit including an adjustable resistance. Another adjustable resistance is placed in parallel with the two capacitances. The two adjustable resistances cause distortions with opposite signs which combine in such a manner as to produce a symmetrically distorted output ramp with a substantially S-shaped development, increasing in a straight line at the centre, more slowly at the two minimum and maximum ends.
This type of correction has the drawback of being complicated to adjust and not compatible with automatic adjustment systems based on the use of control voltages.